<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #loading-btn {
      display: flex;
      align-items: center;
      justify-content: center;
      height: 100vh;
    }

    button {
      background: transparent;
      border: 0;
      border-radius: 0;
      text-transform: uppercase;
      font-weight: bold;
      font-size: 20px;
      padding: 15px 50px;
      position: relative;
    }

    button:before {
      transition: all 0.8s cubic-bezier(0.7, -0.5, 0.2, 2);
      content: '';
      width: 1%;
      height: 100%;
      background: #ff5964;
      position: absolute;
      top: 0;
      left: 0;
    }

    button span {
      mix-blend-mode: darken;
    }

    button:hover:before {
      background: #ff5964;
      width: 100%;
    }
  </style>
</head>

<body>
  <div id="loading-btn">
    <button><span>Hover me</span></button>
  </div>
</body>

</html>